The Invention Relates to the Field of Rotary Kilns
Many processes performed by rotary kilns require heat to raise the temperature of a material or assist in a particular chemical reaction, where the atmosphere inside the kiln is not involved in the reaction. However some reactions do require interaction with the atmosphere. Oxidation/reduction reactions are included in this category. In the specification, we will disclose a particular example of removing combustible contaminants from foundry sand for the purpose of beneficial reuse.
However, the method disclosed in the following specification may be applicable in other applications. Instead of foundry sand, the granular burden material may be a fuel such as sawdust, coal, or coke breeze. The kiln becomes a furnace. The kiln can also become, under reducing conditions, an ore processor. Non-redox reactions like nitriding power metal would benefit from the incorporated designs.
In a simple rotary kiln, i.e. one that has no flighting, only the surface of the sand is exposed to the kiln's atmosphere. Sand grains below the surface are “smothered” by the grains lying between their position and the surface, as oxygen trying to reach the subsurface sand would require transmission through a torturous path. Products of combustion would experience the same difficulty when traveling in the opposite direction. This condition makes the combustion process very inefficient. Long retention times would be required.
To improve this process, kiln manufacturers have incorporated flighting mechanisms or flights within the kiln for containing and lifting a portion of the burden and drop it progressively as the kiln rotates, creating a “waterfall” or “cascading” effect. This “waterfall” effect allows the burden to fall through the atmosphere, providing enhanced contact or interaction between the burden and the atmosphere. Although this is an improvement over the simple kiln, it is not a substantial improvement. If a snapshot would be taken at any point in time, it would reveal that only a small portion of the sand is in freefall. The majority of sand is still in a smothered condition, either lying on the kiln wall, or lying on the flights themselves. Minor improvements can be made by the proper choice of type of flights used, along with the number of them positioned around the circumference, and their depth in relation to the depth of the burden. However, the benefit of these improvements still leaves much to be desired.